There is actually quite a bit of confusion regarding words that begin with the prefix bi- (biweekly, bimonthly, etc.). For example, biweekly means both every other week, and twice a week. Similarly, bimonthly means every other month and twice a month. In the publishing industry bi- words usually take the every other connotation, while in education bi- words usually take the twice per period connotation (Source: Merriam Webster’s English Usage Dictionary).
As writers it is up to us to ensure clear communication. MW’s English Usage Dictionary points out that often writers expect that readers will automatically know which usage the writers intend, and therefore they don’t clarify their meaning. However, if you are working in HR, and you tell employees they are going to be paid bimonthly, you better be sure your employees understand whether you mean they will get paid 24 times a year, or six — it makes a big difference.
I’m a volunteer writer for a non-profit organization that recently introduced a new feature that allows users to set e-mail digest notifications to be sent weekly, bimonthly, or monthly. I asked the developer what they meant by bimonthly; in this case, bimonthly means twice a month.
The rule of thumb: for clarity don’t use bi- words if you can avoid them. Instead of bimonthly, we’re changing our application to say semimonthly, which is more clear. If you really need to use a bi- word, it is best to provide either contextual evidence to indicate your meaning, or spell it out in parenthesis after the first usage of the word, as in: This bimonthly publication (published six times annually)…
Now you know.